(CNN) -- Thirty-five years ago, two Americans landed on the moon, taking the human race to another celestial body for the first time.

Neil Armstrong and Buzz Aldrin, piloting the Eagle landing module, touched down in the moon's Sea of Tranquility, on July 20, 1969. A third U.S. astronaut, Michael Collins, was in the orbiting command module overhead.

"That's one small step for [a] man, one giant leap for mankind," said Armstrong.

The words were seared into the memories of the 600 million some people -- about 1/5 of our planet's population -- who watched the broadcast transmitted from the lunar surface.

"For one priceless moment in the whole history of man, the people of this Earth are truly one," said President Richard Nixon.

Accolades poured in from around the world. New Zealand Prime Minister Keith Holyoake said: "The impossible is only that which takes a little longer to do."

And for a time it seemed the impossible was just the beginning.

Ambitious space experts and political leaders planned lunar colonies, space stations and Mars expeditions. For them, the footprints left by the astronauts in the soft, gray lunar soil were merely baby steps toward a greater destiny in the cosmos. Man would finally become a space-faring species.

"The federal government was seen as key toward innovation and technological progress, by defining what's essentially civilian, like the space program, as a government endeavor," said Allan Needell, curator of Apollo exhibits at the National Air and Space Museum.

However, that vision of exploration clashed with troubles at home.

In the summer of 1969, political and social turmoil ravaged the United States. Many struggled to come to grips with the assassinations of Sen. Robert F. Kennedy and civil rights leader Martin Luther King. Riots rocked some of the nation's largest cities, while rising casualties and horrific images out of Vietnam fueled anti-government demonstrations.

Yet for eight days, the Apollo 11 mission transcended those concerns.

"There were racial, economic and social divisions -- strains that were very real and very important," said Needell. "[The moon landing] was the culmination of forces that brought everyone together to pursue a common goal."

'The Eagle has landed'

On July 16, Armstrong, Aldrin and Collins climbed into the Apollo. The unit's Saturn V rocket, equipped with the most powerful engines ever built, blasted them into space.

Their trajectory took them around Earth and toward the moon when a third-stage rocket fired. Two modules -- the Eagle, for landing, and the Columbia, a command and service center -- entered the moon's orbit July 19.

Armstrong and Aldrin left several footprints in the soft, gray lunar soil.

A day later, Armstrong navigated a course that included a potentially deadly crater and boulder field before successfully touching down in flat terrain.

"Houston, Tranquility Base here," he said. "The Eagle has landed."

Armstrong, soon joined by Aldrin, spent 21 hours on the lunar surface. The two sampled rocks, surveyed terrain, set up experiments and famously planted an American flag in the soil. A plaque they left read: "Here men from the planet Earth first set foot upon the Moon. July 1969 AD. We came in peace for all mankind."

The pair then re-entered the Eagle, reuniting with Collins and the Columbia module before returning home by splashing down in the Pacific Ocean.

Within six months, the United States had landed a second team of astronauts on the moon. For the next three years, at approximately six-month intervals, NASA sent more manned missions to the moon. The sixth and final one, Apollo 17, left the moon December 14, 1972 -- the last time humans set foot on another celestial body.

Path to the moon

Apollo 11, and the six subsequent lunar missions (except for Apollo 13, during which U.S. astronauts averted disaster but returned home without setting foot on the moon), marked a stunning climax after years of tribulations and smaller successes.

The U.S. space program, under the Pioneer and Ranger missions, made repeated attempts to hurl a satellite toward the moon starting in 1958. The first 10 U.S. robotic missions to the moon failed due to booster rocket misfires, faulty computers and other malfunctions.

On July 28, 1964, Ranger 7 finally succeeded. The craft beamed 4,316 images back to Earth before crashing on the lunar surface. More fly-bys and reconnaissance missions followed, paving the way for Apollo 11.

Aldrin descends the ladder of the Eagle landing module before touching down on the moon.

Armstrong, Aldrin and Collins' mission was a stunning victory for the United States in the so-called "space race" with archrival the Soviet Union.

By 1961, Soviet scientists had launched the first satellite (Sputnik I) into orbit, sent the first animal (a dog named Laika) into space, and made the first human spaceflight with Yuri Gagarin's Earth orbit.

Less than ten months after Alan Shepard became the first American in space -- making a 15-minute, 28-second suborbital flight -- the United States finally matched Gagarin's feat on February 20, 1962, when John Glenn orbited the Earth aboard the Mercury-Atlas 6 capsule.

But, by then, America had been issued a challenge.

"I believe that this nation should commit itself to achieving the goal, before this decade is out, of landing a man on the moon and returning him safely to Earth," President Kennedy told Congress in 1961. "If we are to go only halfway, or reduce our sights in the face of difficulty, in my judgment it would be better not to go at all."

He asked for $531 million in 1962, at least $7 billion over the ensuing five years and the nation's determination to prove America's scientific superiority.

"In a very real sense, it will not be one man going to the moon -- if we make this judgment affirmatively, it will be an entire nation. For all of us must work to put him there," he said. "For while we cannot guarantee that we shall one day be first, we can guarantee that any failure to make this effort will make us last."

Congress and the American people rose to the challenge. NASA quickly regained lost ground, mounting manned missions in space by 1961 and was making final preparations for a lunar landing by 1968.

"Basically, we upped the ante," said Needell.

Apollo's legacy

In January 2004, President Bush unveiled an ambitious plan to return Americans to the moon by 2020.

"Mankind is drawn to the heavens for the same reason we were once drawn into unknown lands and across the open sea," said Bush. "We choose to explore space because doing so improves our lives and lifts our national spirit."

But this time Americans did not embrace the plan, and critics claimed funds for the costly initiative should be spent on domestic programs.

The astronauts planted a U.S. flag on the moon, but did not claim the body as American territory.

This response, said Needell, speaks to the unique factors that led to the Apollo missions' success -- as well as the fact that humans haven't returned to the moon in nearly 32 years.

"After the Apollo program, NASA was not able to sustain that level of commitment, because the impetus was no longer there," he said. "We're a decentralized country with lots of interests, and it's very difficult to maintain that focus or direction."

The vast resources devoted to the Apollo program produced spectacular results, but such largess was not to be repeated, relegating the lunar missions to part of NASA's "Golden Age," according to space historian Roger Launius.

The space shuttle fleet, launched with great hope of opening up space, has proved even more expensive than the Saturn rockets that propelled astronauts to the moon. In an ironic twist, the shuttle fleet's grounding following the Columbia accident has forced American astronauts to fly Russian Soyuz rockets developed at the height of the space race.

Nearly three decades later, visitors still flock to the Apollo-related exhibits at the National Air and Space Museum in Washington, D.C., reflecting the inquisitive spirit Bush alluded to: the desire to investigate, learn and discover new things and places.

"Many people saw [the Apollo landing] as adding to the tradition of Western expansion and the new frontier," said Needell. "There is a strong cultural thread in America seeking exploration and fulfilling man's destiny."